European Patent 0 116 636 discloses a positioning device for determining the relative position of two parts movable relative to one another. One of the two parts has a gap-free sequence of binary code marks of equal length. Each n sequential code marks form a binary code word of n places; each code word to be formed thus occurs only once. A sequence of code marks of this kind can be produced as a binary pseudo-random sequence with the aid of a feedback shift register.
A code scanner is provided on the other part, capable of detecting at least n sequential marks. A plurality of sensors is provided for this purpose for each code mark.
In this known position indicating apparatus, one code word defines a particular relative position of the two parts to one another. The length of the code marks measured in the direction of motion and the number of maximum possible code words define the maximum length of the path that can be addressed with code words.
The accuracy with which the position can be measured, or in other words the resolution, depends on the length of the code marks, unless further measures are taken. For the sake of accurate positioning, this length must therefore be selected to be as short as possible.
If the known positioning apparatus is used for crane ways or for overhead conveyors, the entire travel path has to be addressed. This means code words of many places, or digits. The maximum travel path is essentially limited by the sum of the lengths of all the code marks appearing in the pseudo-random sequence, even if there are many path segments in which the accurate position of the particular vehicle need not be known.
If the travel path is to be lengthened, then either the place number n of the code words, or the length of the code marks, must be increased. The first provision increases the expense for apparatus for reading the code words, or requires a higher order production polynomial for the code mark sequence; the latter provision reduces the resolution capacity.